科研之友
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摘要
The nature of the unusual cation-pi interactions between cations (H+, Li+, Na+, Be2+ and Mg2+) and the electron-deficient B=B bond of the triplet state HB=BH ((3)Sigma(-)(g)) was investigated using UMP2(full) and UB3LYP methods at 6-311++ G(2df, 2p) and aug-cc-pVTZ levels, accompanied by a comparison with 1:1 and 2:1 sigma-binding complexes between BH and the cations. The binding energies follow the order HB=BH center dot center dot center dot H+ > HB=BH center dot center dot center dot Be2+ > HB=BH center dot center dot center dot Mg2+ > HB=BH center dot center dot center dot Li+ > HB=BH center dot center dot center dot Na+ and HB=BH ((1)Delta(g))center dot center dot center dot M+/M2+ > H2C=CH2 center dot center dot center dot M+/M2+ > HC=CH center dot center dot center dot M+/M2+ > HB=BH ((3)Sigma(-)(g))center dot center dot center dot M+/M2+. Furthermore, except for HB center dot center dot center dot H+, the sigma-binding interaction energy of the 1:1 complex HB center dot center dot center dot M+/M2+ is stronger than the cation-pi interaction energy of the C2H2 center dot center dot center dot M+/M2+, C2H4 center dot center dot center dot M+/M2+, B2H2 ((1)Delta(g))center dot center dot center dot M+/M2+ or B2H2 ((3)Sigma(-)(g))center dot center dot center dot M+/M2+ complex, and, for the 2:1 sigma-binding complexes, except for HBBe2+center dot center dot center dot BH, they are less stable than the cation-pi complexes of B2H2 ((1)Delta(g)) or B2H2 ((3)Sigma(-)(g)). The atoms in molecules (AIM) theory was also applied to verify covalent interactions in the H+ complexes and confirm that HB=BH ((3)Sigma(-)(g)) can be a weaker pi-electron donor than HB=BH ((1)Delta(g)), H2C=CH2 or HC=CH in the cation-pi interaction. Analyses of natural bond orbital (NBO) and electron density shifts revealed that the origin of the cation-pi interaction is mainly that many of the lost densities from the pi-orbital of B=B and CC multiple bonds are shifted toward the cations.
